(i) Field of the Invention
This invention relates to asphalt production and particularly but not exclusively to production of asphalt used in road making.
(ii) Prior Art
Asphalt making apparatus usually includes a conveyor system, transporting mineral ore aggregate from cold feed bins to a dryer, where it is preheated. The dryer is usually of the rotatable drum type fitted with lifters to ensure good contact with the burner combustion gases. A conveyor system transports the preheated mineral ore aggregate from the dryer to a vibrating screen, which screen separates the preheated aggregate into various fractions and deposits the different fractions into hot bins. A mineral filler system discharges specified weights of mineral ore aggregate from the hot bins into a pug-mill mixer or similar mixing device, and after a specified dry mixing time, a predetermined weight or volume of bitumen is discharged into the pug-mill, the added bitumen being held in electrically heated storage bins prior to discharge into the pug-mill. The pug-mill then wet mixes preheated mineral ore aggregate and bitumen until a uniform mixture is obtained, and the resultant asphalt is then discharged into trucks and transported to the road-making site. Asphalt produced in this way is known as "hot mix".
One reason for producing "hot mix" in factories remote from the road-making site is that bitumen is an inflammable material and careful control must be maintained over heating of the bitumen to maintain the bitumen in a stable state. The required control would be more difficult to achieve if the "hot mix" was produced on site. Also, the equipment needed to produce the "hot mix" is not easily transportable and therefore moving the equipment from site to site is difficult and inefficient.
The burners used in the described drum type dryers usually operate on diesel fuel oil, or in some cases on heavy furnace oil, and are fitted with an exhaust fan to remove combustion gases and provide suction for secondary air to the burner. A duct positioned prior to the fan is fitted with a separator to effect dust removal, the dust being discharged as waste. Where the factory is located in built-up areas, pollution controls are necessary to limit the amount of dust discharged into the atmosphere to within "clean air" regulations.